Originally posted by MMVIII It's all about the grip and how comfortably the fingers can operate the controls.
Specifically (Google translate): "The joystick provided on the back is a focusing point lever, but the shape of the surface that the thumb touches is devised.
It is necessary to make sure that it is caught firmly so that it does not slip when operating with the thumb, but considering only the goodness of the finger grip, there is a problem that the protrusion gets stuck in the finger and feels pain. Therefore, the shape is changed between the central protrusion and the peripheral protrusions to achieve both good finger grip and comfortable use without feeling any pain."